July 22, 1897] 



NATURE 



283 



I adopt the main divisions of the former articles, viz. Towers, 

 Apparatus, Lamps. 



There is not much to be noted in regard to the architecture 

 or engineering features of the iron and stone lighthouses erected 

 in Britain during the past ten years. The principal island 

 towers are in the Shetlands, viz. Stroma, Fair Island, and 

 Sule Skerry, which is one of the dangerous reefs known as 

 skerries so thickly strewn on the northern and western coasts. 

 These all are the successful works of Messrs. D. and C. 

 Stevenson. The Trinity House has not erected any rock or 

 pile tower since the Round Island in 1887. 



Of lightships there have been established two only, which are 

 at the mouth of the Thames to mark the Edinburgh Channel, 

 and, like all the Trinity lightships, they have 21 -inch silver- 

 plated reflectors, with the powerful two-wick oil burners of Sir 

 J. Douglass, the flash from one face of the light being equal to 

 20,000 candles. 



To nearly all the light-vessels in our waters sound-signals, 

 chiefly of the siien type, have been added, which are invaluable 

 in the frequent sea fogs that beset our shores, since by a 

 marvellous beneficence of nature — for the proof of it we are 

 chiefly indebted to the researches of Tyndall — fog that quenches 

 light deepens the intensity of sound. The siren is now an 

 indispensable adjunct to our principal land-lights, and a visit to 

 the Isle of May, the St. Catherine's, or Ailsa Craig, would well 

 repay any one desirous of seeing the latest and best arrange- 

 ments for the production and transmission of its weird and 

 inimitable notes. 



Many of our land lights and floating lights have been con- 

 nected with the main telegraphic system "for life-saving pur- 

 poses only.'' There were, at the end of 1896, 27 such stations 

 in England and Wales, 14 in Scotland, and 11 in Ireland. 

 The most suitable places were selected for the observation of 

 passing vessels and for the immediate transmission of reports of 

 casualities to the nearest points available for help. It is by no 

 means an easy work to make a durable telegraphic connection 

 with a lightship, and with all stations the cost is considerable ; 

 but the benefit is so marked that this communication, long and 

 urgently demanded, will soon be extended to all other sites on 

 the coast where the advantage is evident, and the difficulty not 

 insurmountable. 



Of the new sea lights in the decade under review may be 

 mentioned the Girdleness, the Rattray Head, the Tarbetness, 

 the Stroma, the Scaddon, the Skroo, and the Sule Skerry, all 

 on the Scottish coast ; and the Round Island, the St. Catherine's 

 electric, the Spurn Point, and the Withernsea, on the English 

 coast. The characteristics of several older lights of fixed sec- 

 tions have been changed, such as the Whitby, Coquet, Orford- 

 ness, Southwold, and Needles, which have been made occulting 

 lights. 



To complete the bare statistics of the subject, it may be 

 mentioned that the whole number of lights of all sorts and 

 sizes on the coasts of the United Kingdom on December 31, 

 1896, according to the Admiralty List, was 1095, an increase of 

 al)out 200 over the number of ten years ago. Of the 1095 

 lights, not more than 11 or 12 per cent, may be classed as of 

 sea-power, and 6 or 7 per cent, as lightships. The rest are 

 port, harbour and pier lights, showing a very abundant supply 

 of local signals apart from the great coast signals controlled . by 

 the three Lighthouse Boards. 



It would seem, indeed, that the programme of coast lighting 

 is almost completed — at least as regards England. A powerful 

 nil light is to be erected by the Trinity House in St. Mary's 

 Isle, on the north-east coast, and two others in Lundy Isle in 

 the Bristol Channel, where also, at Lynmouth, an electric light 

 is proposed. A further improvement in the fine reflector light 

 of Beachy Head is also spoken of. The Northern Commis- 

 sioners will erect a first-class light at Noup Head, in the 

 Orkneys, and another on the Flannan Islands in the Hebrides. 

 I'or Ireland no new light appears to be intended for the 

 l)resent ; but the improvement of the Fastnet, the most out- 

 lying of our lights westward, must soon be taken in hand. 



The lantern which crowns the tower and protects the appa- 

 ratus of a land or rock lighthouse has been materially improved 

 in its construction. The perfect lantern is of adequate area to 

 contain the light and accessory parts, and afford convenient space 

 for service. Its diameter for a sea light is rather 14 feet than 

 12 feet. It is of circular form throughout, having a cast-iron 

 pedestal of sufficient height to carry an inside and an outside 

 gallery for cleaning purposes. The framing is of gun-metal or 



NO. 1447. VOL. 56] 



wrought iron. The half-inch plate-glass of the purest quality, 

 while resisting wind and weather to a great extent, intercepts 

 the least possible light from the lenses. There is provided 

 abundant ventilation to sustain the central lamp, and refresh the 

 keepers. The best contribution of the Trinity House to the 

 structure has been the large cylindric ventilator on the double 

 copper dome, by which the last-named advantage is mainly 

 secured. 



Turning to the optical features of the decade, let us see what 

 changes have affected the two main factors of illumination — the 

 fixed light and the revolving light. The ever-increasing number 

 of ships and ports, and the sustained demand for greater and 

 greater speed of vessels, and for sea signals that shall serve more 

 and more to meet all conditions of weather and to discriminate 

 harbours and channels of approach, have stimulated the efforts 

 of lighthouse engineers in the two directions of power and 

 distinctiveness. 



The characteristics introduced of late years have been con- 

 fined for the most part to new combinations and periods of 

 group flashes, or of single equal flashes. Mixed lights of fixed 

 and revolving sections, and therefore with unequal degrees of 

 visibility, are no longer in good repute, although, in the French 

 service especially, they are still retained. Colour also is being 

 gradually abandoned for sea lights. For harbour lights, how- 

 ever, it is still freely resorted to, most of all in Northern 

 Europe, as in the Omo light, Baltic Sea, where in one small 

 apparatus there are no fewer than six characteristics with 

 seventeen variations. 



Power has been sought — 



(i) By the substitution, wherever practicable, of annular for 

 cylindric lenses, or of revolving for fixed sections. 



(2) By the longer focal distance and larger condensing surface 

 of these lenses, and by superposing them in vertical series. 



(3) By using an illuminant of maximum intensity, whether 

 oil, gas, or electricity. 



(4) By certain variations and extensions of the Fresnel 

 refractors, giving a higher coefficient of beam. 



(5) By invoking the principle of the full perception of the 

 light from an annular lens moving in rapid rotation with very 

 short intervals between the flashes. This is known as the 



fen klair. 



(6) By rotating an opaque screen around a lamp, or by rais- 

 ing and lowering quickly an opaque cylinder, or by cutting off 

 and relighting of gas in rapid alternation. 



Let us consider these in order. 



(i) The reasons for the disuse of fixed sea lights, except in 

 the case of marking or intensifying particular sectors, are chiefly 

 their want of power to penetrate to their natural horizon, and 

 the increasing number of bright fixed lights on ships and on 

 shore, which may in certain conditions lead a navigator into 

 danger from the difficulty of distinguishing them from a light- 

 house. The flashing light, with its superior power and 

 numerous possible distinctions, is the best safeguard against this. 

 (2) The hyper-radial lens of 1330 millimetres focal distance, 

 first suggested (as Prof. Tyndall declared) by Mr. John Wigham, 

 and concurrently at least by Mr. Thomas Stevenson, who less 

 fitly termed it hyper-radiant (a good name for the ten-wick oil 

 flame, or ten-ring gas flame), has quite justified the expectation 

 of lighthouse engineers as a convenient and powerful instrument 

 admirably adapted to the large burners now employed. This 

 lens, like its predecessor of 920 millimetres radius, is commonly 

 used without catadioptric prisms, as in the Spurn Point light. 

 The disuse of prisms effects a great saving in cost at but an in- 

 considerable diminution of power where a high vertical angle 

 of refraction is adopted. Yet it must be admitted that this is 

 at the expense of symmetry and elegance. The French light 

 on Cape d'Antifer is a fine example of the hyper-radial system, 

 embracing a full complement of prisms. 



There has also been constructed, at Mr. Wigham's sugges- 

 tion, as an experiment, what he aptly calls a giant lens of 2000 

 millimetres radius, but it is not yet adopted in practice. 



The superposition of lens lights, each with its own burner, is 

 an obvious means of gaining power, and it is remarkable that, 

 although first suggested in 1859, it was not practically utilised 

 with dioptric apparatus until 1872. Whether biform, triform, 

 or quadriform arrangements be resorted to, the advantage of 

 concentrating or reducing the intensity of the beam according to 

 atmospheric variations is inestimable, and the adaptation reflects 

 undoubted credit on Mr. John Wigham, whose polyannular gas- 

 burner on a like plan had been already approved. The triform 



